1. Field of the Invention
This invention relates to ignition devices, particularly for internal combustion engines.
2. Prior Art
Operating a spark ignited internal combustion engine so as to achieve lower inherent engine emissions tends to approach the limits of the capability of conventional ignition sources to avoid misfire when using a lean air/fuel ratio and to tolerate an increased amount of exhaust gas recirculation. Therefore there has been interest in developing new ignition sources and determining their effects on engine performance and emissions.
Among the high energy ignition systems investigated is plasma jet ignition. Such a system is described in Society of Automotive Engineer' Paper No. 770355, 1977 entitled "Design of a Plasma Jet Ignition System for Automotive Application" by J. R. Asik, P. Piatkowski, M. J. Foucher and W. G. Rado. A plasma jet spark plug has a plasma jet cavity which can produce spark energy. If a sufficient amount of electrical energy is delivered to the plasma cavity in a short enough period of time, a plasma torch or a jet is generated that protrudes momentarily out of the end of the cavity. This plasma consists of free electrons and ions that are at a high temperature (10,000.degree. to 30,000.degree. K.) and are therefore highly energetic and chemically active.
The plasma is produced by the instantaneous heating of the gas confined in the cavity by the electrical energy. This raises the temperature of the confined gas and produces partial ionization of this gas. The sudden increase in temperature also raises the instantaneous pressure of the partially confined plasma, causing a portion of it to be ejected out of the end of the cavity. There are many factors that can influence the operation of the plasma jet plug. Some of these factors are the amount of applied electrical energy, the rate of energy delivery, the volume of the cavity, the cavity dimensions, the cavity orifice size, the ambient gas pressure, and the quantity of fuel present in the cavity. Among the parameters that characterize the plasma jet are the length and diameter of the luminous region or plume, the turbulence generated by the jet, and the instantaneous temperature profile of the jet.
U.S. Pat. No. 3,521,105 to Franks discloses an ignition device with planar, parallel electrodes. A pulse of ionizable gas is passed between the electrodes which are energized by a high-voltage power supply. The ionized gas is injected into the combustion chamber of an internal combustion engine to cause the air/fuel mixture to ignite.
U.S. Pat. No. 3,842,818 to Cowell et al discloses a plasma jet ignition device for an internal combustion engine. The device generates a plasma flame and employes two voltage sources. The higher voltage causes electrical breakdown across a spark gap so that the lower voltage source can discharge across the gap.
U.S. Pat. No. 3,842,819 to Atkins et al also discloses a plasma jet ignition device which employs two voltage sources.
U.S. Pat. No. 3,911,307 to Goto et al discloses a spark plug which generates and injects a plasma-like gas into the air/fuel mixture in an internal combustion engine.
However, much of this earlier work on a plasma jet ignition system has shortcomings such as high electrode erosion, high radio frequency interference, possible electrical shock hazard, and an estimated high system cost. These are some of the problems this invention overcomes.